The present invention relates to a process control apparatus and, more particularly, to a process control apparatus for controlling a process whose dynamic characteristic changes.
In order to control a temperature, a pressure, a flow rate or the like of a plant, control parameters of the process control apparatus should be properly set in accordance with the dynamic characteristic of the plant to be controlled. When the control parameters do not correspond to the dynamic characteristic, an oscillation in the process control loop of the plant may occur and part or all of the plant may be destroyed. An operation environment for the plant during operation changes in accordance with a tendency of energy conservation. For example, the plant is operated with energy conservation, so that the dynamic characteristic of the plant often changes greatly.
There are two types of conventional control apparatuses: one is an apparatus in which control parameters are fixed during the operation of the plant; and the other is an apparatus in which the control parameters are tuned by detecting the dynamic characteristic during the operaton. In the former apparatus, the control parameters fall within a safety range in consideration of the worst conditions. However, when the change in the dynamic characteristic is greater than the expected value, the control characteristics are degraded. In order to prevent this, the latter apparatus is proposed to tune the control parameters during the operation. This process control apparatus is classified into two types in the following manner.
When a relationship between the cause and effect of a change in the dynamic characteristic (i.e., a relationship between a cause of a change in the dynamic characteristic and the resultant dynamic characteristic) is known, a gain schedule control system is used, as shown in FIG. 1. In this system, an auxiliary signal directly related to a change in a dynamic characteristic is generated from a process 10 and is supplied to a gain scheduler 12. The control parameters are calculated in accordance with a gain schedule curve stored therein. The control parameters are then supplied to a controller 14. The controller 14 controls a control signal u(t) such that a process output signal y(t) becomes equal to a set point signal r(t).
However, when the relationship between the cause and effect is unknown, a model reference adaptive control system shown in FIG. 2 is used. This system has a process 10 and a reference model 16. A set point signal r(t) is supplied to the reference model 16 as well as to a controller 14. An error, i.e., an output error between the outputs from the process 10 and the reference model 16, is calculated by a subtractor 18. An adaptive tuning device 20 determines the control parameters of the controller 14 in such a manner that the output error becomes zero.
These two conventional control systems are summarized as follows. The gain schedule control system can be used only when the relationship between the cause and effect of the change in the dynamic characteristic of the process 10 is known and the auxiliary process signal directly related to the change in the dynamic characteristic can be detected. Therefore, this system cannot be used when the dynamic characteristic of the process 10 is unknown, resulting in inconvenience. On the other hand, the model reference adaptive control system can be used even if the dynamic characteristic of the process 10 is unknown. However, it takes a long time for this system to tune the control parameters of the controller 14 in accordance with the dynamic characteristic of the process 10. In addition, when the process 10 is a nonminimum process, it is difficult to control based on the reference model 16. When unknown disturbance or measuring noise is present, the adaptive tuning device 20 erroneously detects a change in the dynamic characteristic. As a result, the control parameters of the controller 14 are erroneously changed. This problem has not been solved until now.
These conventional process control systems for tuning the control parameters during the operation of the process are highly sensitive. The sensitivity of the control system is defined as follows. When the dynamic characteristic of the process changes from G.sub.p (s) to G.sub.p (s) (=G.sub.p (s)+.DELTA.G.sub.p (s)), and the transfer function (y(t)/r(t) in FIGS. 1 and 2) of the control system as a whole changes from T(s) to T(s) (=T(s)+.DELTA.T(s)), the sensitivity S(s) is given by EQU S(s)=(.DELTA.T(s)/T(s))/(.DELTA.G.sub.p (s)/G.sub.p (s)) (1)
Equation (1) indicates how a change in the process to be controlled influences the control system. The smaller the sensitivity becomes, the less the degradation of control performance of the control system, with respect to the change .DELTA.G.sub.p (s) in the process, becomes. On the contrary, a control system of high sensitivity is defined as a system in which the transfer function of the control system as a whole greatly changes in accordance with a change in a dynamic characteristic of the process. A change in a transfer function of the control system degrades the control characteristic of the control system. A control system of low sensitivity is defined as a robust control system in which the transfer function of the control system as a whole does not change, thus preventing degradation of the control performance even if the dynamic characteristic of the process to be controlled changes.